1. Field of the Invention
The present invention relates to an electrophoretic particle, a process for its production, and its use.
2. Description of the Related Art
There is a phenomenon called electrophoresis, in which charged particles dispersed in a liquid migrate in the liquid in response to an external electric field, and much attention has been paid to display devices making use of this phenomenon, referred to as “electrophoretic display devices.” The electrophoretic display devices fall under the category of the non-light emitting display devices, and have many excellent properties such as extremely lower power consumption, a higher level of visibility (wider viewing angle), and capability of recording displayed contents for a long term without consuming electricity, when compared with liquid crystal displays.
For example, Japanese Patent Publication No. 50-15115 discloses a cell-type electrophoretic display device in which a dispersion containing at least two kinds of electrophoretic particles having difficult optical reflection properties and different charge polarities is interposed between a pair of transparent substrates each having a surface on which a transparent electrically conductive film is formed. This electrophoretic display device is such that these particles are allowed to cause electrophoretic migration by the application of a prescribed voltage between the transparent electrically conductive films to optically hide one kind of particles from the other kind of particles, and utilize a consequent change in optical reflection properties of the dispersion for displaying or recording character data or image data.
The cell-type electrophoretic display device, however, has a serious problem that electrophoretic particles may unevenly be distributed to cause a deterioration in display quality. Thus, there has been proposed a method of disposing porous spacers between the transparent substrates and injecting a dispersion into voids formed by the spacers. This method, however, has a serious problem that it may be quite difficult to evenly inject a dispersion into a great number of voids and, in the case of a dispersion using a volatile dispersing medium, the properties of the dispersion may be changed during injection work, it may be difficult to ensure uniform display quality.
For solving the above problems of the cell-type electrophoretic display, for example, Japanese Patent Application Publication No. 64-86116 discloses a microcapsule-type electrophoretic display device in which many microcapsules are disposed between a pair of electrode plates, at least one of which is transparent, each microcapsule encapsulating a dispersion containing electrophoretic particles in a dispersing medium colored with a dye, these electrophoretic particles having an optical property different from that of the dispersing medium. This electrophoretic display device is such that the encapsulation of a dispersion containing electrophoretic particles in many microcapsules prevents uneven distribution of the electrophoretic particles, and has greatly improved display properties such as display responsiveness, display stability, contrast, and rewritable times of display, when compared with the cell-type electrophoretic display device.
In recent years, the electrophoretic display devices have drawn much attention to themselves, in addition to their excellent display properties, because they can be applied to flexible display devices which are thin and can freely be bent like a paper or display devices which can easily be produced to have a great surface area at a low cost. Further technical development has been proposed in the field of “digital paper (electronic paper)” such as paper-like display and rewritable paper, and applications have been tried in the display on IC cards and IC tags, as well as electronic white boards, direction boards, advertisement boards, electronic newspapers, electronic books, and mobile terminals (e.g., PDAs).
However, the conventional electrophoretic display devices have not yet had sufficiently satisfactory display properties when compared with the required level of performance both at the present time and in the future, and for application to various display devices as described above, in particular, it has been necessary to further improve display properties such as display responsiveness and contrast.